Method for applying a phosphate coating to iron and steel

ABSTRACT

Disclosed is a method and composition useful for applying a phosphate coating to an iron or steel surface. The surface is contacted with an aqueous solution containing acidic zinc phosphate, chlorate ion and between 0.1 and 10 mg/1 vanadium, calculated as V2O5, at a temperature of less than 65* C.

United States Patent 1191 Schumichen et a1.

METHOD FOR APPLYING A PHOSPHATE COATING TO IRON AND STEEL Inventors:Helmut Schumichen, Frankfurt;

Gerhard Muller, Hanau, both of Germany Oxy Metal Finishing Corporation,Warren, Mich.

Filed: Aug. 31, 1972 Appl. No.: 283,446

Assignee:

Foreign Application Priority Data Sept. 2, 1971 Germany 2143957 US. Cl.106/14, 148/615 Z Int. Cl C09d 5/12 Field of Search 106/14; l48/6.15 R,6.15 Z

References Cited UNITED STATES PATENTS 9/1958 Kronstein 148/615 RFOREIGN PATENTS OR APPLICATlONS 495,098 2/1937 Great Britain 148/6.l5 Z

Primary ExaminerLorenzo B. Hayes Attorney, Agent, or FirmArthur E.Kluegel; Marvin Trimas; B. F. Claeboe 5 7 ABSTRACT Disclosed is a methodand composition useful for applying a phosphate coating to an iron orsteel surface. The surface is contacted with an aqueous solutioncontaining acidic zinc phosphate, chlorate ion and between 0.1 and 10mg/ l vanadium, calculated as V 0 at a temperature of less than 65 C.

9 Claims, No Drawings METHOD FOR APPLYING A PHOSPHATE COATING TO IRONAND STEEL BACKGROUND AND SUMMARY OF THE INVENTION The invention pertainsto an improved method for applying a phosphate coating to iron and steelusing an aqueous zinc phosphate solution containing acid chlorate.

It has long been known that acid zinc phosphate solutions containingchlorate as accelerator can be used for applying phosphate c'oatin gs toiron and steel. Without the use of additional oxidizing agents,.solutions of this type lead to thin, hard, dark coatings, which providevery good protection against corrosion when combined with organiccovering layers. At operating temperatures below 65 C, however, thechlorate alone is not sufficiently reactive to oxidize the divalent ironentering into the solution immediately and quantitatively to trivalentiron, which is then immediately deposited as insoluble iron l'lIphosphate. In the absence of other oxidizers which are capable ofoxidizing the divalent iron with sufficient speed even at bathtemperatures below 65 C, the divalent iron concentration, especially athigh throughputs, increases rapidly. For example, during the treatmentof morethan 0.04 m surface/liter/day, the concentration of divalent ironin the bath solution rapidly increases to several 100 mg/l. However,this produces disadvantages. In particular, these considerablequantities of divalent iron present in the solution, upon cessation ofthe passage of iron surfaces through the bath solution due to atemporary production stoppage or during standing overnight, are slowlyoxidized to trivalent iron by the oxidizers in the solution such aschlorate and oxygen from the air, and precipitated as insoluble iron lllphosphate. This reaction greatly increases the acidity of the solution,since the phosphate anions are precipitated as iron (Ill) phosphate, butthe hydrogen cations are left behind in the phosphoric acid solution.When iron surfaces are again treated in this phosphate treatmentsolution after such stoppages in operation, only thin or passive layersare produced at short phosphation times because of the excessively highacidity. Tobe sure, it is possible to obdesirable phosphate coatingsagain be precipitatedwithin short times. This measure is an additional,troublesome process, which can only be performed by trained personnel.

In order to avoid this disadvantageous accumulation of divalent ironsolution, it has long been known that it is possible to use otheroxidizing agents, in addition to the chlorate, which are capable ofimmediately and quantitatively oxidizing the divalent iron which entersinto solution to trivalent iron even at temperatures below 65 C, e.g.,nitrite (see, for example, British Pat. No. 551,261).

The use of nitrite in addition to chlorate, to be sure, represents ahighly effective procedure with respect to the removal of the iron whichgoes into solution, and the related possibility of maintaining constant,short treatment times; however, the use of nitrite also entails 2procedural disadvantages, e.g., the necessity of waste water processingand of removingnitrogen-containing gases from the waste air. The nitrousgases can also lead to corrosion at the transmission points before andafter the treating zone. In addition, the continuous analyticalmonitoring of the nitrite content in the bath solution entails a certainexpense. It has been found that zinc phosphate coatings which areapplied from solutions containing chlorate and nitrite provide poorercorrosion protection in combination with an organic covering layer thancorresponding coatings produced with a chlorate-containing butnitrite-free solution.

Since the coatings applied using chlorate-containing zinc phosphatesolutions, because of their quality and low layer weight, represent aparticularly suitable base for subsequent electrophoretic painting,considerable interest exists in improving this method while avoiding thedisadvantages linked with the additional use of nitrite.

The object of the invention is now a method for applying a phosphatecoating to iron and steel with the aid of a chlorate-accelerated zincphosphate solution at temperatures below 65 C, which is characterized bythe fact that a zinc phosphate solution containing 1-10 g/l C10 and 0.1to 10 mg/l vanadium, calculated as V 0 is used.

' It has surprisingly been found that the addition of the smallquantities of vanadium to the chlorate bath eliminates the disturbancesoccurring at operating temperatures below 65 C, without increasing thetreatment time and without the simultaneous use of nitrite or othercustomary oxidizers which are capable of immediately and quantitativelyoxidizing the divalent iron which goes into solution at the intendedtemperatures. The procedure according to the invention is thereforesimpler and easier to carry out. In addition, it exhibits the advantagethat the applied phosphate layers display the low layer weight desiredfor subsequent electrophoretic painting and advantageous hardness,without the decrease in corrosion protection observed with the use ofnitrite occurring.

The vanadium can be added to the chloratecontaining zinc phosphatesolution in accordance with the invention in the form of vanadiumpentoxide (V 0 or in the form of other vanadium compounds. It' can alsobe added to the concentrate used in making up the solution. The vanadiumcontent of the treating solution preferably amounts to 0.4-7 mg/l,calculated as V 0 The range of 0.4-2.7 mg/l is particularly suitable.The significance of the vanadium content is independent of the valenceof the vanadium actually present in the solution. Contents of more than10 mg/l can interfere with layer formation by passivation of thesurface.

The zinc phosphate solution used advantageously contains about 1 to 10g/l zinc, corresponding quantities of phosphate and l to 10 g/lchlorate. It can also contain nitrate and other customary additives,such as nickel, calcium, simple or complex fluoride, boron compounds andthe like. The further use of nitrite is undesirable because of thedisadvantages mentioned above.

The iron and steel surfaces to be provided with the coating can bebrought into contact with the treating solution in any desired manner,e.g., by immersion in the solution or spraying with the solution. Thespray method is advantageously used. The spraying time advantageouslyamounts to a maximum of 150 seconds. Somewhat longer contact times maybe required in the immersion process.

The method according to the present invention is advantageously carriedout in the temperature range of 40 to 65 C. At temperatures above 55 C,the treating bath also remains essentially free from divalent iron evenat higher throughputs. At temperatures below 55 C, certain quantities ofdivalent iron can enter the solution, without an appreciableaccumulation occurring.

TABLE 1 V Minimum Expericoncen- Temperphospha- Layer m/l Iron (11) mentNo. tration ature, tion time weight treated content mg/l C g/m surfacemg/l The maximum quantities of Fe (11) which occur in the solutionamount to about 85 mg/l. They do not lead to the interference mentionedin the preceding.

The replenishment of vanadium in the solution used in accordance withthe present invention can be easily accomplished by adding 0.044 mgvanadium, calculated as V 0 to the solution per square meter of treatedsurface.

Supplemental zinc, P 0 and chlorate is provided in the customary manner,to provide point constancy or corresponding to use. The vanadiumcompound may be added to the supplemental concentrate if desired.

To be sure, it was already known from German Pat. No. 1,078,845 thatoxidizing agent-containing solutions of layer-forming phosphates couldbe supplemented with up to 1 g/l vanadium, calculated as V0 in order toachieve a considerable increase in the weight of the layer in comparisonto vanadium-free baths. The weight of the layer was supposed topractically double at an identical treatment time. This effect wasdescribed, based on the example of nitrateand nitrate-nitrite-containingzinc phosphate baths at temperatures of 98 C in the immersion process or65 to 70 C in the spray process. It should also be noted that the effectof the desired layer increase was taught to be practically independentof the operating temperature, and that chlorate, among others, couldalso be used as accelerators. However, thin, hard phosphate coatingswith layer weights of only 1.6 to 2.5 g/m are specifically desired aspretreatments prior to electrophoretic painting. Therefore, aconsiderable prejudice existed against the use of the vanadium additivewhich was known to increase the layer thickness. It is entirelyunexpected for the addition of small quantities of vanadium to a zincphosphate solution containing 1-10 g/l of chlorate as accelerator and attreating temperatures below 65 C, to be suitable for eliminating thedifficulties mentioned above, without increasing the layer weight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A treating solution wasprepared, which contained 3.1 g/l Zn, 8.3 g/l P 0 5.2 g/l C10 13.8 g/lN0 and 0.1 g/l Ni. Sheets of steel were treated by spraying attemperatures of 53 C and 60 C. In accordance with It is obvious that theaddition of vanadium has resulted in a marked reduction in the iron (11)content of the solution, and that the layer weight of the coating wasscarcely affected by this addition.

The corrosion behavior of the coatings produced at 60 C in accordancewith the invention (Experiment No. 4) was likewise compared withcoatings obtained from vanadium-free solutions, containing onlychlorate, and containing iron (11) (Experiment No. 2) and the coatingsproduced at 60 C from solutions containing chlorate and nitrite(Experiment No. 5). The solution used in Experiment No. 5 contained 3.1g/l Zn, 8.3 g/l P 0 5.2 g/lC10 13.8 g/l NO;,, 0.1 g/l Ni and 0.11

g/l NaNO The phosphated sample panels were first electrophoreticallypainted, then provided with a dual-layer lacquer coating. The salt spraytest was carried out according to ASTM B 117-64. The results obtainedare summarized in Table 2.

It is obvious that the use of the nitrite-free, vanadium-containingsolution in accordance with the invention leads to coatings which offercorrosion protection at least percent better than coatings produced withthe aid of the nitrite-containing solution.

What is claimed is:

l. A method for applying a phosphate coating to an iron or steel surfacecomprising contacting said surface with an aqueous solution comprisingacidic zinc phosphate, chlorate ion and from 0.1 to 10 mg/l of vanadium,calculated as V 0 at a temperature of less than 65 C.

2. The method of claim 1 wherein the chlorate ion content of thesolution is between 1 and 10 g/l.

3. The method of claim 2 wherein the zinc content of the solution isbetween 1 and 10 g/l.

4. The method of claim 3 wherein the vanadium is present in an amount offrom 0.4 to 7 mg/l, calculated as V205.

5. The method of claim 4 wherein the vanadium is present in an amount offrom 0.4 to 2.7 mg/l, calculated as V205- 6 g/l chlorate and from 0.1 to10 mg/l vanadium, calculated as V 0 8. A method of maintaining theferrous ion content of a chlorate accelerated acidic zinc phosphatesolution for the treatment of iron or steel below a maximum of about 85mg/l when the solution is below 65 C comprising including from 0.1 to 10mg/l of vanadium, calculated as V 0 9. The method of claim 8 wherein thechlorate content is from ll0 g/l and the zinc content is from l-lO g/l.

2. The method of claim 1 wherein the chlorate ion content of the solution is between 1 and 10 g/l.
 3. The method of claim 2 wherein the zinc content of the solution is between 1 and 10 g/l.
 4. The method of claim 3 wherein the vanadium is present in an amount of from 0.4 to 7 mg/l, calculated as V2O5.
 5. The method of claim 4 wherein the vanadium is present in an amount of from 0.4 to 2.7 mg/l, calculated as V2O5.
 6. The method of claim 1 wherein said surface is contacted with said solution for a period of time sufficient to produce a coating of layer weight between 1.6 and 2.5 g/m2.
 7. An aqueous composition useful for pretreating an iron or steel surface prior to electrophoretic paint deposition comprising acidic zinc phosphate, from 1 to 10 g/l chlorate and from 0.1 to 10 mg/l vanadium, calculated as V2O5.
 8. A method of maintaining the ferrous ion content of a chlorate accelerated acidic zinc phosphate solution for the treatment of iron or steel below a maximum of about 85 mg/l when the solution is below 65* C comprising including from 0.1 to 10 mg/l of vanadium, calculated as V2O5.
 9. The method of claim 8 wherein the chlorate content is from 1-10 g/l and the zinc content is from 1-10 g/l. 